1. Field of the Invention
The present invention is directed to a redundancy structure of a flash memory, and in particular to an erase verifying apparatus in a serial flash memory having a column redundancy, and a method thereof.
2. Description of the Background Art
As the present invention relates to a project in progress, there is no referential one among the conventional flash memory-related arts which have been manufactured or registered. Especially, a serial access flash memory employing an NOR type array and a column redundancy has not been disclosed.
Currently, among the flash memories, only a random access flash memory using an NOR type array structure, a cell array and an input/output buffer being directly connected(a data buffer is not used for connection), such as Intel and AMD, employs a column redundancy. According to the U.S. Pat. No. 4,701,887, it is disclosed a memory-related technique (Video RAM) carrying out a serial access by using the data buffer and employing the column redundancy. However, an erase verifying operation is not described therein.
According to the U.S. Pat. Nos. 5,347,484 and 5,438,546, it is disclosed a replacement method of the column redundancy in the random access flash memory which does not include the data buffer. The above patents use a hardware method directly transferring replacement information to each sense amplifier or multiplexer through a wire.
The hardware method can also be utilized in the random access flash memory which includes the data buffer. Here, as a number of the sense amplifiers or a volume of redundancy (a number of the redundancies to be replaced) is increased, a circuit is more complicated, that is a problem may occur in interconnection.
In the random access memory RAM, a number of the sense amplifiers is generally identical to a number of input/output I/O lines (.times.8, 8 and .times.16, 16). However, in the case of the serial flash memory, at least 64 to 128 or 256 sense amplifiers are required, or sense amplifiers as many as the columns (2048 or 4096) are necessary for system performance. Accordingly, it is impossible to employ the hardware method transferring the redundancy information to the plurality of sense amplifiers via the wire.
In the Nor type serial flash memory in which the data buffer is positioned between the cell array and I/O buffer, a cell data is not accessed directly from an external space but accessed through the data buffer. A specific column in a main cell array is replaced by the redundancy column. When the column is completely replaced, a cell is erased and an erase verifying operation is carried out.
According to the conventional erase verifying method, the erase verifying operation can be exactly performed when the information (current flow) that a current column is replaced by the redundancy column is transferred to the sense amplifier. However, the conventional erase verifying method has a disadvantage as follows.
It is presumed that, a cell is in an erase state when a current flows in the cell (data "1"), and the cell is in a program state when the current does not flow in the cell (data "0"). When the current column is replaced by the redundancy, an output of the sense amplifier always displays the program state of the cell (namely the current does not flow in the cell) regardless of the cell state during the erase verifying operation, and thus it cannot be informed of whether the cells are exactly erased. Here, the current column is replaced by the redundancy and not employed, As a result, the output from the sense amplifier is useless information. Accordingly, the erase verifying operation cannot be exactly carried out merely with the output from the sense amplifier.
Thus, in order to ignore the output from the sense amplifier and regard the current cell as being erased, the information that the currently-verified cell is replaced by the redundancy should be transferred to the sense amplifier or an external erase verifying apparatus. As described above, the replacement information can be transferred to all the sense amplifiers through the wire according to the hardware method. However, in case the sense amplifiers are increased more than a predetermined number, the hardware method itself cannot be employed. As a result, it is impossible to transfer the replacement information.